Comparison of CHF Enhancement on Microstructured Surfaces With a Predictive Model

Abstract

An experimental investigation on the role of surface structure in pool boiling critical heat flux (CHF) enhancement on randomly roughened and structured surfaces is considered. In the first set of experiments, CHF was measured on a horizontal circular brass surface with root mean square (RMS) surface roughness varying from 0.15 to 5 μm using pentane, hexane, and FC-72 as working fluids at system pressures varying from 150 to 450 kPa. CHF is observed to increase with increasing surface roughness, although the enhancement diminishes with increasing pressure. A maximum enhancement factor of 1.15 is observed for the surface with 5 μm RMS roughness. CHF enhancement was measured for a second set of experiments with FC-72 and hexane on new reentrant interconnected microstructured hoodoo surfaces, and hoodoo sizes range from 10 to 80 μm. The measured CHF enhancement factor on the hoodoo surfaces varies from 1.05 to 1.67. The enhancement generally increases with decreasing hoodoo size, and the maximum enhancement factor (1.67) is for hexane with 10-μm hoodoos. The maximum enhancement factor for FC-72 is 1.48, also with 10-μm hoodoos. It has been demonstrated that the surfaces which show good CHF enhancement exhibit excellent wicking properties. A recent lift-off CHF model for pool boiling is modified to account for the wicking action observed for structured surfaces. Using the measured wicking rate for various surface/fluid combinations, the lift-off model gives reasonably good prediction of the measured CHF enhancement.